This invention relates to apparatus for testing a fault indicator of the type which has surge-current detection means and settable fault indicating means.
The fault indicators tested by the apparatus of this invention are widely used in electric power distribution systems. In the event of a power line fault in the system, maintenance personnel have to track down where the fault occurred. Fault indicators are intended to facilitate this task.
In a typical system, each of a plurality of power transmission lines has serially connected to it a separate fault indicator. If a short circuit occurs somewhere in a line, a surge current results and the detection means of the fault indicator in that line senses this surge current and in response sets the fault indicating means of the fault indicator accordingly. A customary fault indicating means comprises a circular display face showing at least one white letter "N" (for Normal) against a dark background. It also includes a rotatable flag assembly that rotates incident to fault detection so as to cover the letter "N" and to provide a display of a dark letter "F" (for Fault) against a white background. In tracing the system to locate a fault, the maintenance personnel check the fault indicators one-by-one to see which one has been set to indicate a down-line fault.
It has been found that commercial fault indicators are not fully reliable. One problem is that, when a short circuit develops, sometimes a fuse blows or a circuit breaker trips before the fault indicator responds. Thus, the current, although having peaked, drops to zero before the fault indicator has had time to register the down-line fault. This can lead to considerable confusion because the maintenance personnel will inspect the fault indicator and find that it hasn't registered a fault and then will be misled into looking elsewhere to trace out the fault. In view of this problem, there is a real need for equipment that maintenance personnel can use either in a central repair facility or on site for advance checking of whether the fault indicators will work effectively when needed.
In the past, manually controlled testing techniques have been used for testing fault indicators. According to the conventional approach a variac is connected to the fault indicator under test and the variac is manually adjusted until a very high current flows through the fault indicator. With this technique certain operational parameters of a static nature can be determined. In particular, a determination can be made of the magnitude of the sensed current necessary to trip the fault indicator so that it indicates a fault. This checking technique as to static operational parameters does not adequately simulate actual field conditions. That is, it does not simulate the pulse nature of the peak currents that occur incident to a real fault. Thus a too slowing reacting fault indicator can pass this test and yet not operate effectively in the field.